The present invention relates to apparatus for non-invasively measuring one or more blood parameters. More specifically, the invention relates to apparatus for the transcutaneous measurement of vascular access blood flow (xe2x80x9cTQAxe2x80x9d) that is capable of generating accurate TQA measurements, even when the volume of access being measured is extremely small in size or extremely deep or when the access is of varying nature, such as a synthetic or native fistula. Further, it is possible to infer additional information about the access area, such as collateral veins or competing vessels.
Access blood flow for hemodialysis patients can now be measured non-invasively through a novel photo-optic transcutaneous technique as described in co-pending application Ser. No. 09/750,122, filed Dec. 29, 2000 (which is incorporated herein by reference in its entirety), using a transcutaneous TQA sensor as disclosed in application Ser. No. 09/750,076, filed Dec. 29, 2000 (which is also incorporated herein by reference in its entirety), and more particularly, the transcutaneous TQA sensor described in connection with FIGS. 2-6 thereof (hereinafter, xe2x80x9cthe prior art linear sensorxe2x80x9d).
With reference to FIGS. 1, 2, and 2A, the prior art linear sensor 10 includes two light emitting sources (emitters) 12a and 12b, preferably light emitting diodes (LEDs) of specific wavelengths, and two complementary silicon photodiode detectors 14a and 14b alternatingly arranged in a straight line at identical intervals to form three LED/detector pairs with identical separations between the members of each pair, for the purpose of measuring the bulk absorptivity (xcex1) of the volume immediately surrounding and including the access site A, and the absorptivity (xcex1o) of the tissue itself. The LEDs preferably emit light at a wavelength of 805 nm-880 nm, because it is near the known isobestic wavelength for hemoglobin, is commercially available, and has been shown to be effective in the optical determination of whole blood parameters such as hematocrit and oxygen saturation.
The technique is accomplished by directly placing the prior art linear sensor 10 on the skin of a patient with the aligned emitters 12a and 12b and detectors 14a and 14b perpendicular to the vascular access site A, and measuring the back-scattered light from a turbid tissue sample to determine the percentage change in hematocrit xcex94H as a bolus of saline passes through the access vessel.
When the prior art linear sensor 10 is placed on the surface of the skin, each LED 12a and 12b illuminates a volume of tissue T, and a small fraction of the light absorbed and back-scattered by the tissue and red blood cells is detected by its adjacent photodetector 14a or 14b, which generates a detection signal. When the volume of tissue illuminated includes all or even part of the access A, the resultant xcex1 value includes information about both the surrounding tissue T and the access itself. In order to resolve the signal due to blood flowing within the access A from that due to the surrounding tissues T, the prior art linear sensor 10 illuminates adjacent tissue regions T on either side of the access A. Values of xcex1o for tissue regions T not containing the access A are then used to normalize the signal, thus providing a baseline from which relative changes can be assessed in access hematocrit in the access blood flowing directly under the skin. The intensity of the signal produced by each photodetector 14A or 14B is proportional to the total absorption and scattering within a given volume of tissue between each detector 14a or 14B and its adjacent LED 12a or 12b. During saline dilution, only the hematocrit inside the access A varies, and the detected signal changes are solely dependent upon the optical property changes within the small volume of access viewed by the sensor 10.
By correcting the signal in the volume containing the access A with the average reference signal in the volumes without access, the sensor 10 provides a signal solely dependent on the hematocrit flowing in the access. Then, traditional Ficke principle mathematics can be used to calculate the blood flow rate using the following equation:       Q    a    =      V          ∫                                    Δ            ⁢                          xe2x80x83                        ⁢                          H              ⁡                              (                t                )                                                          H            a                          ⁢                  ⅆ          t                    
For a given separation between LED and photodiode in the sensor 10, the volume of tissue illuminated and viewed by the prior art linear sensor 10 is relatively constant and the signal-to-noise ratio of this technique depends on the volume of access included inside the tissue volume. When the volume of access included inside the tissue volume is small enough due to extremely small size or excessive depth, the signal-to-noise ratio falls to a level that would not generate accurate measurement results. It would accordingly be desirable to improve the signal-to-noise ratio so that accurate measurements can be taken even when the access is extremely small or very deep.
According to W. Cui (xe2x80x9cPhoton Diffusion Theory and Noninvasive Tissue Optical Property Measurement,xe2x80x9d PhD. Thesis, Biomedical Engineering Department, Rensselaer Polytechnic Institute (1990)), the principle path of diffused photons in a turbid medium is in the gradient direction of the photon density distribution, which is perpendicular to the contour surfaces. Along this direction, photons consistently travel all the way from the LED to the detector in a curved path. In a later study, W. Cui et al. (xe2x80x9cExperimental Study of Migration Depth for the Photons Measured at Sample Surface,xe2x80x9d SPIE, Vol. 1431, pp 180-191 (1991)) further showed that the photon flux path from LED to detector has a xe2x80x9cbananaxe2x80x9d shape that reaches deepest into the tissue at the mid-portion of the xe2x80x9cbanana.xe2x80x9d More significantly, in this xe2x80x9cbananaxe2x80x9d-shaped photon path, there is a region in the middle between LED and detector near the tissue surface that is totally outside the detected photon flux path. This means that anything in this region will not interact with the photons that reach the detector and will never be xe2x80x9cseenxe2x80x9d by the detector. This finding was verified by S. Feng et al. (xe2x80x9cMonte Carlo Simulations of Photon Migration Path Distributions in Multiple Scattering Media,xe2x80x9d SPIE, Vol. 1888, pp 78-89 (1993)), using both analytical perturbative diffusion theory and Monte Carlo simulations. This phenomenon also explains the clinical observations that with a visually observable shallow graft, no significant difference in xcex1 is detected with the injection of a saline bolus.
The configuration of the prior art linear sensor 10 allows it (or more precisely, the aligned LEDs 12a and 12b and the detectors 14a and 14b) to be perpendicular to the access A and the photon flux F to travel across the access to generate an illuminated volume of access within the illuminated tissue volume, as shown in FIGS. 1 and 2. For a graft in the center of the photon flux path F, the volume of the access viewed by the prior art linear sensor 10 is limited to the cross-section of the graft and the photon flux path F as indicated by FIGS. 1 and 2. For a graft that is nearly out of the photon flux path F (because it is too shallow, as shown in FIG. 2A, or too deep) the volume of access xe2x80x9cseenxe2x80x9d by the prior art linear sensor 10 is so small that the signal-to-noise ratio is too low to give accurate measurements.
It is to the solution of this and other problems that the present invention is directed.
It is therefore a primary object of the present invention to provide apparatus for non-invasively measuring one or more blood parameters associated with a vascular access, even when the volume of access being measured is extremely small in size or extremely deep.
It is another object of the present invention to provide a sensor for transcutaneous TQA measurement that is capable of generating accurate TQA measurements, even when the volume of access being measured is extremely small in size or extremely deep.
This and other objects of the invention is achieved by the provision of an optical sensor including two pairs of complementary emitter and detector elements, wherein the pairs of emitter and detector elements define two lines at right angles to each other, for the purpose of measuring the bulk absorptivity (xcex1) of the volume immediately surrounding and including the access site, and the absorptivity (xcex1o) of the tissue itself.
In one aspect of the invention, one of the pairs lies to one side of the line defined by the other of the pairs, such that the two pairs of emitter and detector elements form a xe2x80x9cTxe2x80x9d shape.
In another aspect of the invention, each pair of emitter and detector elements comprises an LED of specific wavelength and a complementary photodetector. A wavelength of 805 nm-880 nm, which is near the known isobestic wavelength for hemoglobin, is used.